A typical LCD has the advantages of portability, low power consumption, and low radiation. LCDs have been widely used in various portable information products, such as notebooks, personal digital assistants (PDAs), video cameras, and the like. Furthermore, LCDs are considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
A typical LCD includes an LCD panel, a backlight for illuminating the LCD panel, an inverter circuit for driving the backlight, and a backlight control circuit. The backlight control circuit generally includes a pulse width modulation integrated circuit (PWM IC) for driving the inverter circuit, and a backlight protection circuit for shutting down the PWM IC when any lamp of the backlight has an open circuit or a short circuit.
FIG. 3 is an abbreviated diagram of a typical backlight control circuit used in an LCD. The backlight control circuit 100 includes four load circuits 110, a PWM IC 150, and a backlight protection circuit (not labeled). The backlight protection circuit includes a first transistor 1331, a current limiting resistor 172, and an input circuit 130.
Each load circuit 110 includes a lamp 111 and a lamp inspecting circuit 113 connected in series between a power supply (not shown) and ground. The lamp inspecting circuit 113 includes an output terminal 112. The output terminal 112 provides a high voltage when the corresponding lamp 111 works, and provides a low voltage when the corresponding lamp 111 has an open circuit or a short circuit.
The PWM IC 150 includes a current sampling pin 151. The PWM IC 150 stops working if the current sampling pin 151 has a low voltage.
The first transistor 1331 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The source electrode “S” is grounded. The drain electrode “D” is connected to the current sampling pin 151 of the PWM IC 150. The gate electrode “G” is connected to a power supply via the current limiting resistor 172.
The input circuit 130 includes four diodes 131, four resistors 132, four capacitors 135, a second transistor 1332, a third transistor 1333, a fourth transistor 1334, and a fifth transistor 1335. Each transistor 1332, 1333, 1334, and 1335 includes a source electrode “S”, a drain electrode “D”, and a gate electrode “G”. The drain electrode “D” of the second transistor 1332 is connected to the gate electrode “G” of the first transistor 1331. The drain electrode “D” of the third transistor 1333 is connected to the source electrode “S” of the second transistor 1332. The drain electrode “D” of the fourth transistor 1334 is connected to the source electrode “S” of the third transistor 1333. The drain electrode “D” of the fifth transistor 1335 is connected to the source electrode “S” of the fourth transistor 1334. The source electrode “S” of the fifth transistor 1335 is grounded. The gate electrodes “G” of the second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 are connected to the cathodes of the four diodes 131, respectively. Anodes of the four diodes 131 are, respectively, connected to a corresponding output terminal 112 of the lamp inspecting circuits 113. Each of the gate electrodes “G” of the second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 is grounded via a corresponding resistor 132 and via a corresponding capacitor 135, respectively.
The first transistor 1331, the second transistor 1332, the third transistor 1333, the fourth transistor 1334, and the fifth transistor 1335 are all negative-channel metal oxide semiconductor (NMOS) type transistors.
Operation of the backlight control circuit 100 is as follows. When all the lamps 111 work normally, each of the output terminals 112 provides a high voltage to the gate electrode “G” of the corresponding second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 via the corresponding diode 131. Then the second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335 are switched to an activated state, and the gate electrode “G” of the first transistor 1331 is grounded via the activated second, third, fourth, and fifth transistors 1332, 1333, 1334, and 1335. Thus, the first transistor 1331 is turned off, and the current sampling pin 151 of the PWM IC 150 maintains an original working voltage.
When any one of the lamps 111 has an open circuit or a short circuit, the corresponding output terminal 112 provides a low voltage to the gate electrode “G” of the corresponding second, third, fourth, or fifth transistor 1332, 1333, 1334, or 1335 via the corresponding diode 131. Then the corresponding second, third, fourth, or fifth transistor 1332, 1333, 1334, or 1335 is turned off, so that the gate electrode “G” of the first transistor 1331 is charged to a high voltage by the power supply via the current limiting resistor 172. Thus, the first transistor 1331 is switched to an activated state, and the current sampling pin 151 of the PWM IC 150 is grounded via the activated first transistor 1331. Consequently, the current sampling pin 151 of the PWM IC 150 is charged to a low voltage, and the PWM IC 150 stops working.
The backlight control circuit 100 includes five transistors 1331, 1332, 1333, 1334, and 1335 to carry out the function of protecting the lamps 111. Furthermore, the number of the transistors needed increases along with the number of lamps 111 used in the LCD. Consequently, the cost of the backlight control circuit 100 is high, particularly in the case where the number of lamps 111 is large.
It is desired to provide a backlight control circuit used typically in an LCD which overcomes the above-described deficiencies.